1. Field of the Invention
The present invention relates to a passive sensor which operates on the principle of acoustic surface wave configurations and has sensor signals which can be interrogated by radio.
In many technical applications it is important to provide availability of measured variables that are of interest by wireless means and from a certain distance, i.e. in such a way that the actual sensor element being used operates passively, which is to say it does not require any energy source or power supply of its own. For example, it is of interest to be able to monitor or measure the temperature of the wheel bearings and/or of the brake blocks on a train passing by. Another application is that of measuring the torque of a rotating shaft of a machine. A further major area of application is that of medicine and chemistry, for example being able to establish the partial pressure of oxygen in the blood of a living organism or, in particular in the area of environmental protection, being able to detect concentrations of solvents in air and/or water even from a distance, in order to then have such measured data, for example obtained in a hazard zone, available at a safe remote location and to process that data there.
Previously adopted ways of achieving that objective have been to use active sensors which are fed by battery and are interrogated telemetrically or transmit permanently, or to carry out the monitoring by optical means using a television camera.
Surface wave configurations have been known for almost two decades. Such configurations are electronic-acoustic components which include a substrate with piezoelectric characteristics, at least in subregions of the surface, and finger electrode structures located on or in the surface. Acoustic waves are generated in that surface by electrical excitation, emanating from an electroacoustic (input) interdigital converter. The acoustic waves run in that surface and again generate an electric signal from the acoustic wave in a further (output) converter. An essential feature of those components is that, by selection of the structure of the converters and, if appropriate, of further structures disposed on the surface, a signal processing of the electric signal entered into the input converter, into an output converter signal, can be carried out. Input converters and output converters may also be one and the same converter structure. A signal which is, for example, a broadband radio-frequency signal can be fed to the input and at the output there is available a signal which is, on the other hand, a time-selective pulse-compressed signal, having a timing which is a predeterminable characteristic, dependent on (measured-value) parameters, of the surface wave configuration which is concerned.
Identity tags (ID tags) (known from U.S. Pat. No. 3,273,146 and U.S. Pat. No. 4,725,841), which make it possible for the presence or identity of objects or persons to be established by radio and which operate passively, have operated on the basis of acoustic surface wave configurations for decades. In that case it is a relevant factor that in such a surface wave configuration the interrogation signal can be intermediately stored due to the powerful piezoelectric effect of the substrate and consequently no further power supply of the identity tag is necessary. An electromagnetic radio-frequency interrogation pulse that is transmitted from an interrogation device is captured by the antenna of the surface wave identity tag, i.e. of the ID tag.
Through the use of the electroacoustic interdigital converter, operated as an input, of the surface wave configuration, an acoustic surface wave is generated in the latter. Due to selected structures of the surface wave configuration, which are chosen to comply with respective specifications that can be defined entirely individually, the surface wave generated in the configuration is modulated and, at the output, a correspondingly modulated electromagnetic signal is recovered. That signal can also be received from a distance through the antenna of the configuration. The surface wave configuration consequently responds to the above-mentioned interrogation pulse after a (basic) delay which is specified in advance for the configuration with an (individual) radio-frequency identification code word, that is to be evaluated by radio in the interrogation device concerned. Such a configuration is described, for example, in U.S. Pat. No. 3,273,146 from the year 1966.
Quite independently thereof, it has been known likewise for more than a decade to use sensors operating on the basis of acoustic surface wave configurations such as, for example, a thermometer, pressure sensor, acceleration meter, chemical sensor or biological sensor, etc.. Examples thereof are described in the printed publications IEEE Ultrasonic Symp.
Proc. (1975) pp. 519-522; Proc. IEEE, vol. 64 (1976) pp. 754-756 and Published European Application No. 0 361 729 A2 (published Apr. 4, 1990). Those known configurations operate on the principle of an oscillator, which differs significantly from the mode of operation of the ID tag and, as active configurations, they also require a power supply of their own.
In German Published, Non-Prosecuted Application DE 34 38 051 A1 and U.S. Pat. No. 4,620,191 (Skeie) there is described a passive transponder which is based on a surface wave configuration and responds to an interrogation signal merely with a special response signal that is coded in the surface wave structure and is consequently predetermined and always the same. As a supplement to the above-mentioned printed publications, U.S. Pat. No. 4,734,658 specifies the way in which it is possible to eliminate the investigated temperature dependence of the surface wave configuration used in this transponder, as it is described in IEEE, Ultrason. Symp. 1987, pages 583-585. With a start bit and stop bit, a standardization of all of the propagation times or phase differences is provided. Consequently, the temperature compensation for the known transponder is additionally achieved.
The above-mentioned temperature dependence was investigated in the case of a commercially available identity tag and a linear dependence of the phase difference of the reflected signals of two reflectors of the identity tag on the temperature was established. Additionally specified there is the possibility of use as a surface wave temperature sensor which can be interrogated remotely.
In Rev. of Scient. Instr. Vol. 60 (1989), pp. 1297-1302 there is described a sensor for aerosols which is active, i.e. operated with battery feeding, that operates with surface waves, such as that already known for more than 10 years. The sensor includes two oscillators with energy-fed amplifier circuits which are necessary for their operation.
A further, likewise active, sensor as an electric voltmeter with influencing of the surface wave rate is known from Published European Application No. 0 166 065 A1.
An inductive identification system, which is energy-fed by means of a coupling loop and operates with surface waves, as is used in automation technology, is known from German Published, Non-Prosecuted Application DE 40 25 107 A1. Through the use of evaluation of only error-free signal periods, a reduction in read/write errors is achieved.
With respect to a chirp transform processor that is known for some considerable time and used as a spectrum analyzer, IEEE, Ultrasonic Symp. 1982, pp. 177-179 describes the way in which its temperature dependence can be reduced. One measure specified there is to select an internal operating frequency of the processor in such a way that the otherwise occurring temperature-dependent measuring error of the processor is minimized.